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My latest blog post over on Energy Stats

700 heat pump hot water runs on my 5kW Vaillant Arotherm and 250L Mixergy cylinder.

  • Overnight COP versus Daytime COP (versus off peak electricity)
  • Better COP when reheating from empty?
  • Lots of Graphs!!

Hope you find this useful.

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Do you combine this with battery storage? It would seem like a battery would pay back very quickly in this application with a split tariff.

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I do have a solar and a battery… a deep dive into whole house 2024 costs here if you’re interested

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I been thinking about this with sizing battery storage, on the coldest 20 days (for example) any DHW heating in the daytime will be at high rate as any reasonable sized battery will not cover all daytime usage of a heatpump. Heating DHW when PV is generating can lose 15p per kwh in export payments. Battery power will cost me about 20% over cheaprate due to loses from charge/discharge.

Unless heatpump oversized, heating DHW with heatpump on the coldest days will likely reduce COP for heating as a higher flow temperature will be required to recover the house temperature. But this will not show up in the DHW COP.

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I was expecting more difference between 1pm and 1am. It looks like even with unlimited battery capacity the loses from charger/inverter will be more then the gain from heating DWH in afternoon.

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When on cheap overnight rate and doing two reheats a day, why not set the overnight reheat to a higher target temperature so the daytime reheat is shorter? Both daytime COP and nighttime COP would be lower, but economics and carbon intensity should be better.